A new convenience utility ros-to-csv is now available in snark. It allows to output as CSV the ROS messages from rosbags or from topics published online.
You could try it out, using the ROS tutorial Understanding Topics (http://wiki.ros.org/ROS/Tutorials/UnderstandingTopics):
Run ROS Tutorial nodes:
Run ros-to-csv; then In the shell where you run turtle_teleop_key, press arrow keys to observe something like:
If you log some data in a rosbag:
You could convert it to csv with a command like: